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Anal. Chem. 1985, 57, 1084-1087
Isotopically Selective Resonant Two-Photon Ionization in Supersonic Beams David M. Lubman,* Roger Tembreull, and Chung Hang Sin Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109
Resonant two-photon ionlzatlon In supersonic beams Is used to selectively ionize chlorine and bromine isotopes in large aromatic molecules. The ions are then mass analyzed in a time of filght mass spectrometer as a function of wavelength In order to study changes in the lsotoplc ratios. We have found significant wavelength enhancement of isotopes which appears to be mode speclfk. The enhancement factor in the case of chlorine Isotopes may be on the order of 60 or greater. I n addition, some spectral resolution between the two Isotoplc peaks is posslbie in the case of chiorlne. Isotopic selection has been obtained even wlth the use of relatlveiy low resolution pulsed lasers due to the relatively large vibrational isotope shlfts observed (-2.5 cm-’).
The detection and discrimination of isotopic species in a mass spectrometer depend upon the difference in molecular weight of the isotopic atoms. The ionization source used, generally electron bombardment, provides no selectivity in detecting a specific isotope. However, the possibility of an isotopically selective ionization source may allow for enhancement in the initial ionization and thus detection of a specific component. In particular, we have found the idea of using an optically selective ionization method based upon the unique spectroscopyof molecules to be quite attractive. This is possible based on the idea of producing ions using laser multiphoton ionization spectroscopy as a selective source for mass spectrometry (1-10). Multiphoton ionization occurs when ionization follows the absorption of several photons by a molecule in the presence of an intense visible or UV light source. If the frequency of the laser is tuned to an allowed n-photon transition, then the cross section for ionization is greatly enhanced; if the frequency of the light is not resonant with a real intermediate state, then the probability for ionization is negligible at modest laser energies. The particular process we use in this study is resonant two-photon ionization (RZPI) in which one photon excites a molecule to a real intermediate electronic state and a second photon pumps the molecule above its ionization limit. The necessary conditions for this process to occur efficiently are that the sum of the two photons must be greater than the ionization potential of the molecule and the first photon must be in resonance with a real state. The ionization signal obtained as a function of wavelength will reflect the cross section for absorption of the various rovibronic states in the electronic transition. Thus, although one measures the total ion current, the excitation-ionization spectrum obtained actually reflects the absorption spectrum of the molecule. Thus, R2PI should provide a spectroscopicfingerprint of molecules similar to IR spectrometry;however, R2PI can also provide ions for analysis in a mass spectrometer. The problem with UV-Vis absorption spectrometry of large molecules at room temperature is that their spectra are generally broad and few resolvable spectral features are available for chemical analysis. This is true since at room temperature a large manifold of rovibronic states are populated in the 0003-2700/85/0357-1084$01.50/0
ground electronic state, resulting in broad contour spectra which consist of a congestion of unresolvable rovibronic transitions. However, using the supersonic beam technique which involves a rapid adiabatic expansion of a small percentage of a large polyatomic in a large bath of a light carrier gas, ultracold molecules may be prepared (11,12). In these ultracold species only low internal modes are populated so that in the electronic transition that occurs only a limited number of sharp spectral features are observed. Thus, R2PI can be combined with the supersonic expansion technique to produce “color” selected ions in a mass spectrometer. Isotopic detection of atomic species is in principle possible but often not practical due to very small isotope shifts based upon changes in the electronic zero-point energy as a function of mass. As the mass increases the changes between isotopes become increasingly smaller and separation is only possible with high resolution lasers based upon the hyperfine structure of the atoms. No significant change in isotope ratios was observed in the case of 1’6Lu/176Lu(13,14) using laser ionization with both CW and pulsed laser sources with a 1cm-l bandwidth. In molecules the isotopic shifts are based upon vibrational frequency shifts which are much larger than the electronic shifts. The shifts may become significant especially in cases where the ground-state and excited-state potential wells are shifted so that transitions may occur through several vibrational states. In this case, the contribution of each additional vibrational quantum or overtone is additive. This has been used to obtain significant isotopic enhancement in Iz (15) where transitions from the ground to excited states may provide a transition from u ” = 0 to u’= 22 in the excited state. Iodine though may be a very special case due to its rich electronic structure which allows the accessibility of many different resonances for enhanced isotope discrimination and its absorption is in the visible region and thus is easily accessible with present laser sources. The question examined herein is whether isotopic selectivity can be achieved for a particular atom as part of a larger molecule. This question may be important, for example, for selective detection of medical isotopes that have been used to tag specific molecules. The ability to selectively detect such species in a mass spectrometer may eliminate the need to use radioactive tracers in analysis. Other groups have shown the possibility of selectively detecting the naturally occurring “C isotope in various hydrocarbons using RPPI in supersonic beams (9, 10). Enhanced isotopic selectivity of 13C was achieved using R2PI in supersonic beams in a mass spectrometer although the isotopic shift was not detectable by purely optical spectroscopic methods. In this paper we pursue this idea for isotopes of C1 and Br in order to demonstrate that it is possible to easily detect even heavy atom substituted aromatic compounds with isotopic selectivity in a supersonic beam.
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EXPERIMENTAL SECTION The experimental apparatus has been described elsewhere (2). It consists of a 6 in, liquid N2 baffled pumping station which evacuates a stainless steel six-port cross. The molecular beam 0 1985 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
expands from the top of the cross down toward the diffusion pump, while the laser photoionizationbeam passes through q& windows orthogonal to the molecular beam. The ions produced are then mass analyzed by a time of flight mass spectrometer (TOFMS) which is orthogonal to the laser and molecular beams. The molecular beam is produced by seeding -10 ppm of the polyatomic species of interest into 1 atm of Ar carrier. The expansion proceeds through a 0.05-cm aperture and is pulsed using the Quanta-RayPSV-1 pulsed valve (pulse -55 pa or leas) in order to maintain pressures on the order of 10" torr or less in the TOFMS. The expansion proceeds for 9.25 cm into the acceleration region of the TOFMS where laser photoionization is produced. At this point we are clearly in the "free flow" region (16) so that there are few collisions and the terminal mach no. MT 40 has been reached providing the maximum cooling effect. Further, the absence of collisions should prevent the possibility of isotopic scrambling due to ion/molecule reactions. The TOFMS is based upon the classic diode source design of Wiley and McLaren (17). It has been modified for use with a supersonic beam (5),providing improved resolution by use of the translational cooling effect to minimize the initial energy spread in the acceleration region of the TOF. A dual microchannel plate (R. M.Jordan Co.) was used as the detector so that laser ionization peaks of at least 10-12 ns were observed. Thus, for the case of p-bromophenola resolution on the order of 109was obtained. The resolution is limited by the speed of the oscilloscope (100 MHz). The TOF mass spectrum was signal averaged over 64 pulses using the Biomation 4500 transient wave form recorder (100 MHz, 8 bits). The peak width recorded generally appeared to be 15-20 ns due to pulse-to-pulse instabilities from jitter in the trigger and poorly regulated power supplies. However,the loss in resolution is not important in this experiment since the peaks under study are clearly resolved. The wavelength ionization spectra were obtained by monitoring only the molecular ion of the compounds under study including all isotopic species with the gate of a gated integrator (Stanford Research Systems Model 250). The laser system used was a Quanta-Ray DCR-1A Nd:YAG pumped dye laser system. The WEX-1 wavelength extension device was used to double the frequency of the dye laser in KD*P in order to obtain tunable near ultraviolet radiation. The light was collimated by use of a telescope (positive lens, 30 cm focal length; negative lens, 10 cm focal length) to a -2 mm beam. The total energy was -0.1-0.3 mJ at 10 Hz so that the peak power is -0.5 MW/cm2. The spectral bandwidth is -0.5 cm-l in the
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The p-bromophenol (99%) and 2,6-dichlorotoluene (99+%) were obtained from Aldrich Chemical Co. They were maintained at 180 and 45 OC, respectively in a flowing stream of Ar in order to obtain concentrations on the order of 10-40 ppm in 1atm of Ar. The nozzle was heated to -100 OC in order to prevent condensation in the nozzle.
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WAVELENGTH (nm) Flgure 1. Resonant two-photon ionization (RPPI) spectrum of 23-
dichlorotoluene expanded in a supersonic jet of argon in the region 277-273 nm. The spectrum was obt'ained by monitoring the parent Ion, including the different isotopic species, in a time of flight mass spectrometer. The vertical axis is ionization signal in arbitrary units.
hanced when the molecular beam pulse reaches full "choked flow". In addition, larger clusters appear to need more time to form and thus cluster more efficiently further into the flow-limited plateau of the beam pulse (18). We therefore intersect the beam at its leading edge before clusters have time to form efficiently, yet we are at the beginning of full molecular flow so that the maximum cooling should have been reached. We examined several chlorinated and brominated benzene compounds for isotopic selectivity and in every case observed the ratio of chlorine and bromine isotopes to vary as a function of the wavelength at specific vibrational modes. The natural ratio of 35C1:37C1isotopes is -3:l and of 7QBr:aBr -1:l. In this work, we have chosen to discuss the two examples of 2,6-dichlorotoluene and p-bromophenol. In the case of 2,6dichlorotoluene there are two chlorine atoms and the isotopic abundances are approximately in a 9:61 35C12:35CP7C1:37C12 ratio. The separation factor y achieved for any isotopic separation process is given by Even et al. (9) as
RESULTS AND DISCUSSION The supersonic expansion provided efficient cooling of the polyatomic species as evidenced by the spectrum of 2,6-dichlorotoluene shown in Figure 1where the typical peak width is -0.2 A fwhm. In addition, only a very limited number of sharp spectral features are observed over the 6 nm range illustrated herein as compared to the room-temperature UVVis absorption spectrum which is generally broad and featureless. At the laser intensities employed in these experiments only the parent ion and its isotopes were observed; i.e., no fragmentation was produced. At higher masses several small peaks were observed due to the clustering of M with Ar, where n is the number of Ar atoms in the cluster. However, these clusters do not interfere with the isotope analysis. The total amount of clustering appears to be considerably less than 5% of the total parent peaks. The clustering problem is minimized by delaying the laser pulse so that it interacts with the top of the rising edge of the molecular beam pulse. The laser interacts with only 10% of the molecular beam pulse (2) and this particular point in the beam is optimum for minimizing clustering. This is true since clustering depends on the number of three-body collisions occurring and is en-
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for binary mixtures, where Ci and Cfare the initial and final isotopic abundances. In Figure 2 is shown a wavelength ionization spectrum of 2,6-dichlorotoluene in the region around 273.8 nm. There is a doublet observed and examination of the associated mass spectral peaks shows that the peak a t -273.81 nm is associated mainly with ionization of a molecule with two %C1atoms and the peak at 273.83 nm with a molecule with one 35Cland one 37Cl. The two spectral peaks are not completely resolved; however in previous work with I3C, spectral resolution between two isotopic peaks could not be achieved. It is important to note that significant isotope shifts are seen on only certain spectral peaks although very small, often almost unnoticeable sh& are seen on almost every peak. It might be expected that for the case of benzene rings mass loaded with C1 or Br that anharmonic mixing between the modes would occur and thus no mode-specific isotope shift observed. However, heavy atom substituents introduce distinctive low energy vibrational modes in aromatic molecules that remain somewhat localized and are consistent with a harmonic force field so that large isotope shifts are indeed observed. Such behavior is observed in the spectrum of Figure
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 6, MAY 1985
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273.95 273.9 273.85 273.8 273.75
WAVELENGTH (nm) Figure 2. R2PI spectrum of 2,6dlchlorotoluene in the region around 273.8 nm. The experlmentai conditions are the same as those given in Figure 1.
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Figure 3. Tlme of flight mass spectra of 2,6dlchlorotoluene obtained as a function of wavelength: (A) 273.812 nrn; (B) 273.82, nm; (C) 273.838 nm; (D) 273.85 nm.
1for the peaks at 273.83 and 273.81 nm and for the peak a t 273.57 nm. A similar effect has been seen in the other isomers of dichlorotoluene and also in dichloroaniline, dichlorobenzene, chlorophenol, etc. and also for the brominated analogues studied. In Figure 3 is shown the mass spectrum recorded as a function of wavelength at the molecular ion of 2,6-dichlorotoluene. At 273.812nm we observe almost solely the 35CP5C1 isotope of dichlorotoluene at mass 160. The peak at M + 1 is due to the I3C isotope of the compound. The ratio of the 35C196C1:35C137C1 peak heights is >77:1 as opposed to the usual 9 6 ratio. The enhancement here is therefore at least a factor of 60. The key point though is that this selectivity is based upon the use of relatively low resolution pulsed lasers since the spectral shifta between isotopes of C1 in 2,6-dichlorotoluene are reasonably large, Le., on the order of 0.2 A or -2.5 cm-'.
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WAVELENGTH (nm)
MASS (amu) 160 162 164
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Figure 4. R2PI spectrum of p-bromophenol In the region around 278.75 nm. The spectrum was obtained by monitoring the parent ions, including the dlfferent isotopic species, in a time of flight mass spectrometer.
When the laser is tuned to 273.822 nm, the height ratios between the 35C135Cl:35C137C1 isotopes are now 1.7:l which is close to the natural abundance of 1.5:l. The 37CP7C1peak barely appears and is masked by a background peak in the mass spectrum. The time difference between the two main peaks is 124 ns and the peak width is 15-20 ns. As the laser is tuned to 273.836 nm, the 35CP7C1is enhanced and a small 37CP7C1appears. The ratio of 35CP5C1:35C137C1:37C137C1 is now 1.5:16:1. The enhancement of 35C137C1 over 35C135C1 without considering the 37CP7C1isotope is 16. As the laser is further tuned to 273.85 nm, the 37C137C1 is enhanced at the expense of the other isotopes. The peak ratios are 35Cl~C1:35C137C1:37C137C1 as 1:68. The enhancement of 37CP7C1 over 35C135C1now becomes -64. p-Bromophenol was also examined for isotopic shifts due to the two bromine isotopes 7QBrand *lBr, and once again we observed mode specific isotopic behavior similar to the previous example. As shown in Figure 4, the two bromine isotopes can be spectroscopically resolved where the peak at 278.75 nm shows enhancement of the 79Brpeak while the peak at 278.79 enhances the *lBr peak. However, the isotopic shift is not totally resolvable and as shown in Figure 5 at 278.75 nm the 79Br:81Brratio is only 3.4:l and this is also the enhancement factor obtained. As the laser is tuned to 278.77 nm, the peaks become almost equal in height, which is close to the natural abundance of the two isotopes. The heavy isotope is enhanced over the light one at 278.79 nm by a factor of only -2. The isotope shift, and thus the enhancement in Br, is not as significant as in chlorine. We would like to summarize the significant points as follows: (1) By use of the supersonicbeam technique, sharp spectral features can be observed on which isotopic selective spectroscopy can be performed. (2) The different isotopic species can be partially spectroscopically resolved and a significant isotopic enhancement is observed in a mass spectrometer. (3) The isotopic shifts appear to be mode selective and are probably due to the molecular motions directly coupled to the substituent species. (4) The isotopic selectivity is possible using relatively low
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Anal. Chem. 1985, 57, 1087-1091 '
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mation 4500 transient digitizer and to Jim Jones of Gould, Inc., for technical assistance given during this work.
MASS (amu) 172 174
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LITERATURE CITED
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(1) Lubman, D. M.; Naaman, R.; Zare, R. N. J . Chem. Phys. 1980, 7 2 , 3034. (2) Lubman, D. M.; Kronlck, M. N. Anal. Chem. 1982, 5 4 , 660. (3) Tembreull, R.; Lubman, D. M. Anal. Chem. 1984, 5 6 , 1962. (4) Dletz, T. G.; Duncan, M. A.; Liverman, M. G.; Srnalley, R. E. Chem. mys. Len. 1980, 7 0 , 246. (5) Dletz, T. G.; Duncan, M. A.; Liverman, M. G.; Smalley, R. E. J . Chem. Phys. 1980, 73, 4816. (6) Boesl, U.; Neusser, H. J.; Schlag, E. W. Chem. Phys. 1981, 5 5 , 193. (7) Zandee, L.; Bernstein, R. B. J . Chem. Phys. 1979, 7 0 , 2574. (8) Lichtln, D. A.; Datta-Ghosh, S.; Newton, K. R.; Bernstein, R. B. Chem. Phys. Lett. 1980, 7 5 , 214. (9) Leutwyler, S.; Even, U. Chem. Phys. Left. 1981, 81, 578. (10) Dlmopoulou-Rademann, 0.;Rademann, K.; Brutschy, B.; Baumgartei, H. Chem. Phys. Lett. 1983, 101, 485. (11) Smalley, R. E.; Wharton, L.; Levy, D. H. Acc. Chem. Res. 1977, 70, 139. (12) Anderson, J. 8.; Andres, R. P.; Fenn, J. B. Adv. Chem. Phys. 1966, 10, 275. (13) Miller, C. M.; Nogar, N. S.; Gancarz, A. J.; Shields, W. R. Anal. Chem. 1982, 54. 2377. (14) Mlller, C. M.; Nogar, N. S. Anal. Chem. 1983, 5 5 , 1609. (15) Lubman, D. M.; Zare, R. N. Anal. Chem. 1982, 5 4 , 2117. (16) Lubman, D. M.; Rettner, C. T.; Zare, R. N. J . Phys. Chem. 1982, 86, 1129. (17) Wlley, W. C.; McLaren, I . H. Rev. Sci. Instrum. 1955, 2 6 , 1150. (16) S. Leutwyler, private communication, Basil, Switzerland, July 1984.
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Figure 5. Time of flight mass spectra of p-bromophenol obtained as a function of wavelength: (A) 278.75 nm; (B) 278.77 nm; (C) 278.79
Received for review November 2,1984. Accepted January 16, 1985. We gratefully acknowledge financial support from a Cottrell Research Grant and a University of Michigan Rackham Award. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
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resolution pulsed lasers even for the case of large aromatic molecules.
ACKNOWLEDGMENT Special thanks are due to Gould, Inc., for loan of a Bio-
Quantitative Analysis of Aqueous Species Using Raman Spectrometry and Equilibrium Model Calculations J. A. Sorensen and L. C. Thompson Department of Chemistry, University of Minnesota-Duluth, Duluth, Minnesota 55812 G. E. Glass* Environmental Research Laboratory-Duluth, Duluth, Minnesota 55804
U.S. Environmental Protection Agency, 6201 Congdon Boulevard,
An analytical approach of quantlfylng various chemical species, uslng Raman spectrometry In conjuncllon wlth equlllbrlum modellng, has been tested on aqueous solutlons containing Nd, Cu, and dlplcollnlc acid. Equlllbrlum modeling was used to select optlmum conditlons In slmple solutlons for the determlnatlon of concentration-Raman Intensity relationship. These relatlonshlps were then used to Interpret spectra from more complex solutions and to make comparlsons with equlllbrlum modeling results from the same systems. Peak heights were determlned through curve fitting of the spectra using nonllnear regression and were normallted uslng CI0,as an Internal standard.
The importance of the chemical form(s) of compounds in aqueous solution in the interpretation of ecological impact and 0003-2700/65/0357-1087$01.50/0
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environmental protection has been well established (1-3). However, the direct determination of complexed species in aqueous solutions is a complex problem with numerous experimental difficulties. Among the techniques that have been used are those based on electrochemicalproperties and those based on spectral properties (4). Most common among the latter are the measurements of the ultraviolet and visible spectra. Unfortunately, these are restricted to species that either have strong charge-transfer bands or give colored solutions due to d-d transitions. Moreover, with the transition elements these are not generally useful as diagnostic tools for complexes of a given class of ligands since all the spectra consist of broad bands with very little differentiation. On the other hand, vibrational spectroscopictechniques that use a distinct property of the bonds within the ligand or of the bonds between the metal and ligand could potentially be of considerable value. The most promising of these would
0 1985 American Chemlcal Society
